"N_Cook" wrote in
:

3 of them made on the same batch of tape and on the same machine but
more than 15 years ago. As they are all stored in the same environment
they could theoretically all have chemically degraded/stretched to the
same degree. I'm trying to find some process that gives an independent
verification that does not require a know good machine.
eg quartz controlled (so low tens ppm) f-meter coupled strobe fed to
rotating capstan if diameter/s of the spindles are manufactured to
specific dimensions of precisely known tolerances. Perhaps 1.9 +/- 0.002
mm and 2.4 +/- 0.002mm say, no such data found googling. I can only
measure to +/-0.02 mm and no slip gauges for absolute calibration.
+/-2 in 200 or so, is not accurate enough


Suggestion:

Google for
guitar tuning meter

There are meters available for ~20 bux and even some 'on line' meters.

Record a known good 'string note' frequency and use it for your standard.
Should get you close enough for any audiophile's ears

--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.